On average, healthy adults sleep between six and nine hours per night. The exact amount of sleep required by a person may vary due to a number of factors associated with the person, such as the age of the person, the level of physical activity of the person, the use of alcohol, drugs, and/or medications by the person, and the overall condition of health. Contemporary sleep science distinguishes five stages of sleep (including wakefulness as a pre-sleep stage): a rested wakeful stage, non-rapid eye movement (NREM) sleep stages 1, 2, and 3, and a rapid eye movement (REM) stage. The various stages of sleep may be identified using various techniques, such as monitoring brainwave patterns using an electroencephalogram (EEG) technique, monitoring eye movements using a electrooculogram (EOG) technique, monitoring the movements of the chin using electromyogram (EMG) techniques, and/or other techniques for monitoring the physiological characteristics of a subject.
Rested wakefulness is characterized by low amplitude alpha waves (8-12 Hz) present in an EEG of a subject whose brain waves are being monitored. Alpha waves are brain waves typically exhibited while a subject is in a wakeful and relaxed state with the subject's eyes being closed. The alpha waves typically decrease in amplitude while the subject's eyes are opened or the subject is in a drowsy or sleeping state.
NREM Stage 1 is characterized by irregular theta waves of low amplitude present in the EEG of a subject being monitored and slow rolling eye movements present in an EOG of the subject. NREM Stage 2 is characterized by high frequency (12-16 Hz) bursts of brain activity called sleep spindles riding on top of slower brain waves of higher amplitude. During the NREM Stage 2, a gradual decline in heart rate, respiration, and core body temperature occurs as the body prepares to enter deep sleep. NREM Stage 3 is characterized by delta waves (1-3 Hz) of large amplitude that dominate for more than 20% of the time. Rapid eye movement (REM) sleep presents with a marked drop in muscle tone and bursts of rapid eye movements that can be seen in the EOG. The EEG in REM is not specific and resembles that of wakefulness or NREM Stage 1 sleep. Other physiological signals (e.g., breathing, heart rate) during REM sleep also exhibit a pattern similar to that occurring in an awakened individual.
Sleep stages come in cycles that repeat on average four to six times a night, with each cycle lasting approximately ninety to one-hundred-and-twenty minutes. FIG. 1 illustrates a typical sleep cycle that includes an NREM Stage 1, followed by an NREM Stage 2, followed by an NREM Stage 3, which is followed by a REM stage. The order of the stages of a sleep cycle and the length of the sleep stages may vary from person to person and from sleep cycle to sleep cycle. For example, NREM Stage 3 may be more prevalent during sleep cycles that occur early in the night, while NREM Stage 2 and REM sleep stages may be more prevalent in sleep cycles that occur later in the night. The sequence and/or length of sleep stages (NREM sleep stages 1, 2, 3 or the REM sleep stage) during an (overnight) sleep or (daytime) nap, sometimes interrupted with brief periods of wakefulness, is referred to as sleep architecture.
For optimal results from sleep, a balance between sleep stages is typically required over longer periods of time, such as days or weeks. Sleep deprivation—i.e., the persistent lack of a particular sleep stage (usually REM or NREM Stage 3)—over a period of even a few days can result in the deterioration of cognitive performance of a subject, even if the subject has taken long naps and the total amount of sleep time over the course of each day is relatively normal. For example, a person requiring eight hours of sleep may have only slept six hours each night over a three day period, but may have taken a two hour nap each day. The total number of hours of sleep for each day equals the eight hours required by the person. However, the person may not have experienced sufficient time in one or more particular sleep stages, thereby causing sleep deprivation in the person. Sleep deprivation can affect cognitive performance as well as the physical dexterity of the subject. In addition, the point in a sleep cycle which the subject has experienced just prior to waking can crucially affect the post-sleep dexterity, cognitive performance, and subjective feeling of the subject. For example, a sleeper wakened from late NREM stages 2 or 3 often experiences significant sleep inertia, such as a feeling of grogginess that may persist for up to thirty minutes or an hour after waking.
A large number of people have difficulties with falling asleep, maintaining sleep, experience frequent awakenings, or just do not use their sleep time as well as they could. The effects of even small amounts of sleep loss accumulate over time resulting in a “sleep debt” which manifests itself in the form of increasing impairment of alertness, memory, and decision-making. Vigilance, memory, decision-making, and other neurocognitive processes are all impacted by poor sleep quality, sleep deprivation, and accumulating sleep debt with potentially detrimental consequences. For example, recent National Aeronautics and Space Administration (NASA) technical reports reveal that pilots often experience brief episodes of unintentional sleep while flying. In the general population, chronic sleep loss is increasingly considered a serious public health and safety concern, and impaired vigilance is shown to be a primary contributor to transportation and industrial accidents.
As a practical example, sleep deprivation is particularly problematic among active servicemen. Military operations often combine high-performance demands and significant physical efforts with irregular sleep schedules. Small amounts of sleep loss accumulate over time, resulting in a sleep debt for these individuals. This sleep debt may manifest itself as impairments of cognitive functions and manual dexterity with potentially detrimental consequences in military, as well as civilian, settings.
Many people do not realize they are not sleeping well and are, nonetheless, suffering the consequences of inefficient sleep. Other people attempt to overcome sleep-related problems by taking sleep-inducing or sleep-assisting drugs, such as pharmacological stimulants (e.g., caffeine), attending psychological therapy, using relaxation techniques prior to sleeping, and the like. However, while temporary amelioration of the effects of sleep deprivation can be achieved using some of these techniques, an adequate amount of sleep that is commensurate with a person's accumulated sleep debt is indispensable for complete recuperation in the long run.
Many situations (e.g., in a military context) do not allow for a regular bout of nocturnal sleep. In such situations, brief naps, taken at various times throughout the day, have been advocated as an effective and natural means of countering fatigue and improving performance. Unfortunately, it is not easy to device an optimal schedule for napping, because the effects of a nap on dexterity and cognition depend, not only upon its duration, but also upon the sleep quality, point on the circadian cycle at which the nap occurred, and depth of sleep from which the subject is awakened.
The sleep architecture of a nap is especially important, because various stages of sleep contribute differently to recuperation. Naps composed only of light sleep (NREM Stage 1) do no not improve performance, whereas even a few minutes of solid sleep (NREM Stage 2) boost alertness, attention, and motor performance. Deep sleep (NREM Stage 3) is desirable because of its effects on stress reduction and skill acquisition. However, paradoxically, interruptions of a nap during NREM Stage 3 sleep (e.g., due to an alarm) may lead to decrements in performance as a result of sleep inertia. Adequate balance among the sleep stages over longer periods of time is also important. A persistent lack of, for example, REM sleep, can result in a decline in performance, even if the total sleep time per day appears adequate. Thus, simplified paradigms that only prescribe durations and frequencies for napping will not result in a consistent and effective mitigation of performance deficits.